1. Field of the Invention
The present invention relates to an IC (integrated circuit) socket, and in particular to an IC socket that accommodates a surface-mount IC package such as a ball-grid-array (BGA) package or land-grid-array (LGA) package and is mounted on a printed circuit board so as to electrically connect the integrated circuit of the surface-mount IC package to an electrical circuit on the printed circuit board.
2. Description of the Prior Art
With the increased density of an electronic circuit. large-scale integrated circuits and the like are widely used. Such a high-density device has a large number of lead-out terminals provided on the bottom face thereof in a predetermined pattern and therefore surface mounting has become standard.
With improvements in terminal lead-out shapes and shortening of the pitch between terminals, a BGA-type terminal shape is frequently employed in the case of the pitch being 0.65 mm, 0.5 mm, or 0.4 mm.
FIG. 1A is a side view of an example of BGA-type IC and FIG. 1B is a bottom view thereof. In the BGA-type IC, metal terminals BT are arranged in matrix form in the entire central portion of the bottom surface ICb of a mold portion. Each of the metal terminals is connected as necessary to an inside integrated circuit portion. For mounting, solder balls BL (bumps) are attached to the bottom planar portions Ts of the respective metal terminals BT. At the time of mounting, the IC is placed at a prescribed position on a printed circuit board and is heated in a reflow furnace or the like. As a result, the solder balls BL melt and solder connections are made between the terminals and prescribed portions of wiring patterns on the circuit board (surface mounting).
There is another type of package called a land grid array (LGA) in which a solder material for connection is prepared on a printed circuit board and the bottom planar portions of micro-terminals of an IC package are exposed. An IC package employing BGA or LGA that are slightly larger than the integrated circuit portion may be called a chip size package (CSP).
With a background situation that surface-mount ICs that are expensive and high in integration density and have a very large number of terminals have come to be widely used, there are cases that even a surface-mount IC is attached to a printed circuit board by using an IC socket to protect such an expensive IC from unnecessary mechanical or electrical stress that may occur during a printed circuit board manufacturing process. The use of an IC socket is also meaningful for the purpose of facilitating exchange of an IC that has failed, for example.
In the case of designing and developing a printed circuit board using a high-density component, the high-density component is completed and thereafter the printed circuit board is designed using the completed high-density component. However, this procedure increases the development period and takes long time to complete the printed circuit board having desired performance. To shorten the development period, the printed circuit board is designed parallel with the development of the high-density component. In this case, the test is performed by connecting a number of signal lines to dense terminal lands in a region on the printed circuit board where to place the high-density component. However, work of connecting those signal lines is difficult because it needs to be performed on a very large number of dense connecting points.
Using an IC socket that makes it possible to easily output individual signals to outside and further makes desired electrical connections to the dense lands very easily with high reliability, the development period of a circuit board may be shortened and sufficient trial production and experiment is allowed.
In BGA-type or LGA-type ICs, however, only limited structures are available for an IC socket that can make correspondence to terminal positions that are densely arranged in matrix form. Further, few sockets are available that make it possible to lead out IC terminals to the outside.
FIG. 2 is an exploded side view showing individual components of a conventional IC socket. FIG. 3 is a perspective view showing an appearance of an example of the conventional IC socket that can be used for the above purpose.
Referring to FIG. 2, reference symbol "A" denotes a BGA socket portion for accommodating a BGA IC and reference symbol "B" denotes an adapter board portion for leading out IC terminals. Reference symbol "C" denotes a printed circuit board to which the IC socket is to be attached.
However, the conventional IC socket is disadvantageous in that it is increased in size and hence can be used at limited locations. Further, the conventional IC socket also has a problem that the number of components is large. Furthermore, since the conventional IC socket has PGA terminals for attachment to a printed circuit board, a different printed circuit board needs to be provided for mounting of an actual IC. To deal with digital ICs that operate at extremely high frequencies, connection path lengths need to be shortened and in particular a thin IC socket is required; that is, an IC socket is desired that has a terminal connection structure and a socket structure such that reduction in thickness and sufficient performance can be both achieved.